1. Field of the Invention
This invention generally relates to thermostat temperature control and, more particularly, to a system and method for combining a plurality of physical thermostat controls into a single virtual thermostat for the purpose of controlling peak power usage.
2. Description of the Related Art
Typical electric service to a commercial operation includes billing not only for electric energy usage, but also the peak power demand in each billing period. It is the peak power demand billing that can lead to especially expensive service billing. There exist today energy management systems that manage peak electric demand by forcing energy consumers to lower consumption states and by coordinating their ON-times by schedule. However distributed, when independent heating, ventilation, and air conditioning (HVAC) systems are under independent thermostatic control, there exists a significant probability that the units will be consuming electric power simultaneously, even though their simultaneous operation is not required to maintain the proper environmental temperatures.
For example, a fitness center in an apartment complex has four heat pumps to keep the air temperature at a comfortable level for the tenants. Each heat pump is responsible for a specific zone with in the fitness center. Each heat pump has a separate thermostat that is located in its zone. Three of the four heat pumps consume 1.8 kWh (1 ton) when in the ON mode. One heat pump draws 3.6 kWh (2 tons) when in the ON mode because it is responsible for the zone near the south facing windows that also contains the treadmills.
In the late afternoons during the month of April, the fitness center heat pumps typically have a 25% ON duty cycle. The late afternoon is the typical peak demand period for the facilities common areas, so any reduction in peak power demand is reflected in the April electric billing. When the fitness center's heat pumps are not coordinated, there is a significant probability that multiple heat pumps will be active and the peak electric charge will be unnecessarily increased in response to the demand level of four active HVAC units. Since only a 25% ON duty cycle is needed to maintain the room temperature, there is enough time for each system in sequence to perform its work when the other systems are in their OFF portion of the cycle.
It would be advantageous if the combined peak power to multiple physical thermostats could be efficiently controlled using a central virtual thermostat.